providers/docker: lower priority

We do this mostly because Docker is the only provider that using it
requires some amount of Docker-specific knowledge. VirtualBox, VMware,
etc. kind of "just work". It is not the likely case that someone does
a `vagrant up` and expects Docker, at this point.

plugins/providers/docker/plugin.rb

@@ -16,7 +16,7 @@ module VagrantPlugins
       Docker containers.
       EOF
 
-      provider(:docker, box_optional: true, parallel: true) do
+      provider(:docker, box_optional: true, parallel: true, priority: 3) do
         require_relative 'provider'
         init!
         Provider